1. Field of the Invention
The present invention relates to a lens system changeable between two specific focal length for suitable use with compact still cameras, and, more particularly, to a lens system changeable between two specific focal length for achieving a wide angle photography mode and tele-photography mode, respectively.
2. Description of Related Art
Most lens systems changeable between a wide angle photography mode and a tele-photography mode consist of more than two lens groups. In one type of lens systems of this kind lens groups are moved along the optical axial to achieve focusing from relatively long distances to moderately close distances. Another type of lens systems of this kind includes a converter lens element cooperating with a master lens group for achieving a high ratio of focal length for wide angle photography to for tele-photography. Almost of all of this type of lens systems include a positive power master lens group arranged on the object end. Such a focal length changeable lens system having a master lens group which is moved along the optical axis to achieve a long local length imposes various constraints on the overall size and other relationships of elements within the system including a lens barrel and a shutter which are not always desired. Together, a focusing mechanism is essential to be installed in the camera so as to cooperate with the master lens group. These constrains render compact still cameras difficult to be simple in structure and small in size and additionally, unfavorable to be manufactured at low costs.